In the related art, a technology for detecting a rotation angle of a rotating shaft of a rotor in a multiphase electric motor used for assisting steering of a steering wheel of a vehicle is known. For example, in JP-A-2004-150931, a rotation angle detection device of a motor of which a structure can be simplified without lowering detection accuracy is disclosed. The rotation angle detection device includes a magnet provided at an end portion of the rotating shaft of the motor, a magneto-resistive sensor, and a rotation angle calculation unit, and detects the rotation angle. The rotation angle detection device calculates a rotation speed and rotation acceleration based on the rotation angle, and calculates a speed correction value and an acceleration correction value according to the rotation speed and the rotation acceleration. The detected rotation angle is corrected by the speed correction value and the acceleration correction value, and a corrected rotation angle is calculated.
In JP-A-2016-050841, a magnetic detection device that suppresses manufacturing cost is disclosed. The magnetic detection device is configured to include a magnetic detection unit that detects a change in a magnetic field according to displacement of a detection target and outputs a detection signal having a different phase, a switching unit which is electrically connected to a plurality of magnetic detection units and periodically switches connection with each of the plurality of magnetic detection units, and an operational amplifier for differentially amplifying detection signals which have different phases for each of the plurality of magnetic detection units and are input via the switching unit. In the magnetic detection unit, a permanent magnet is attached to a rotating member such as a steering wheel of a vehicle, and an angle is detected using two sets of magneto-resistive sensors in which four magneto-resistive elements are disposed at 90 degrees.